The general principle of an air seeding system is to dispense seeds and/or other particulate matter (fertilizers, herbicides, etc.) from a hopper or other container into a moving flow of air, where the moving air will carry it through a series of branching tubes to a point where it will ultimately be deposited into the soil. The particulate matter is typically metered in a controlled fashion as it is dispensed from the hopper, allowing the total rate of material distributed to be controlled. However, once the material leaves the hopper, it is difficult to determine precisely which portion takes which specific path through the branching network of tubes to eventually make its way to the end of the seed tubes and be placed into the soil. An air seeding system represents a complex fluid dynamics problem, in which a single initial flow of air and suspended particulate material may be continuously divided and redirected through multiple tubes to manifold towers where it is then split off into branching seed tubes of varying lengths to a point of eventual discharge into the soil. Sharp turns, bends, and forks in the distribution tubes cause restrictions on the material flow, and make balancing the system for even seed and particulate dispersal problematic. A modern air seeder may plant well over 100 rows of seeds simultaneously. If a partial or full blockage develops in one or more of the particulate flow tubes, air flow (and, therefore, particulate flow) increases proportionately in the remaining tubes, further complicating the balancing problem. To optimize the distribution of material and maintain an even balance of distribution, an air seeding system must employ some type of particulate flow monitoring system which measures the amount of particulate material flowing in the distribution tubes (the particulate flow path), a means by which the flow may be adjusted so that an operator can balance the system prior to field use, and a means to detect particulate flow disruption or blockage during use should field conditions cause the system to become unbalanced.
It should be noted that the term “blockage” will be used generally throughout the specification to refer to either a full and a partial blockage in some part of the air seeding system. A partial blockage will still allow some amount of air and material to flow past it, but will reduce the flow noticeably. A full blockage will not allow any material to flow past it (although it may be possible for a small amount of air to leak past a full blockage).
Seed monitoring systems do exist in prior art. One type of seed monitoring system uses optical sensors which detect changes in the amount of light being received from a light source as seeds and other particles pass between the light source and a sensor, blocking light that would otherwise impinge on the sensor. The attenuation of the light received is related to the amount of material passing through the beam. These optical sensors are subject to a number of problems particular to the technology. One such problem is caused by the non-uniformity of the intensity of the light beam used to sense the particles moving in the tube. If the beam intensity is not uniform, and two different but identically shaped and sized particles move through different parts of the light beam, they will produce different results to the receptor, even though they are identical particles. Hence, the amount of material passing in the tube is not directly correlated to the amount of light that reaches the optical sensor, which can result in an incorrect determination of the actual amount of material passing in the tube. In addition, the optical sensors may not be able to detect at all certain material that is moving through locations in the seed tubes that are not covered by the beam. Attempts to create more uniform or more complete light beams to correct these problems have been inadequate or overly complex. As a result, although in optimal conditions optical sensing systems may work well in detecting total tube blockage, they are not very effective in measuring overall material flow, particularly in situations where a great deal of material is flowing in the tubes.
Optical-based seed monitoring systems are also susceptible to problems in normal use caused by a build-up of dust and other foreign matter that can be found inside an air seeder. Often this build up is gradual, causing the sensor to lose calibration over time, becoming less capable of accurately detecting material flow. This build-up can ultimately block sensors and/or light sources completely, causing the system to determine that a large amount of material is moving through the seed tube, or that a blockage has occurred.
Another source of inaccurate readings in an optical seed monitoring system is that when two or more particles happen to line up in such a manner that an imaginary line drawn between them is parallel to the axis of the beam of light from the transmitter to the receiver, one particle can occlude another, causing them to be read as a single particle instead of multiple particles. In other words, one or more particles can hide in the shadow of another particle as they pass through the beam and not have an effect on the quantity of the light reaching the sensor, adversely affecting the ability of the sensor to accurately measure material flow. Further compounding this problem is that not all particles are uniformly shaped (i.e. spherical). An example of one such particle is a wheat seed, which is more or less cylindrical in shape. Such a particle will occlude the beam of light to a greater or lesser extent, depending upon the orientation of the seed as it passes through the light beam. As a result, it is generally not possible to tell with a reasonable degree of accuracy the amount of material moving past an optical sensor in an air seeder.
Another type of material flow monitoring system uses a piezoelectric sensor that is placed into a seed tube such that the moving particles strike the face of the sensor. When a particle strikes the piezoelectric sensor, the sensor is momentarily deformed, causing the sensor to generate a small electrical charge. The magnitude of the electrical charge is detected by an electronic circuit and the particle is counted accordingly.
However, piezoelectric sensors have a number of characteristics that can limit their usefulness when used in an air seeding application or other similar harsh environments. The sensors are high impedance and hence susceptible to interference by strong voltage fields in the environment. This characteristic demands that the sensor be placed close to the detection circuit in order to minimize the effects and occurrences of these fields. This is particularly a problem in an environment like that of an air seeder, where collisions of seeds, dust, fertilizer and other particles generate a large amount of static electricity, and where high electromagnetic field strengths are likely. Placing the sensing electronics in these areas, as dictated by the necessity to place the high impedance piezoelectric sensors close to their electronic discrimination circuitry, exposes them to premature failure as a result of electrostatic discharge and circuit overvoltage.
Another problem with piezoelectric sensors is that the crystal is prone to damage by cracking if overstressed. Because it is desirable to produce a signal strong enough to overcome the background noise inherent in an air seeding system, and because the amplitude of the signal generated by the sensor is directly proportional to the deflection of the crystal, piezoelectric sensors are often placed such that maximum deflection of the crystal is achieved, which requires location directly in the path of the material flow such that some material will strike the sensor pad with maximum impact. Being so placed, the piezoelectric sensors are subject to damage through impact and abrasion in being struck by numerous, large, or fast moving particles in the air stream (small stones, for example) over a prolonged period of time. Further complicating matters, even though these sensors are placed directly in the path of material flow, they are characteristically struck by a relatively small portion of the total material flowing in the sensor tube, and hence are incapable of accurately measuring the total amount of material flowing in the line due to the relatively small amount of material that actually strikes the sensor.
In yet another embodiment, the piezoelectric sensors are designed such that a pin is attached perpendicular to the surface of the sensor. In this design, the piezoelectric crystal is not placed directly in the path of the material, which helps to mitigate damage which may be caused by material directly striking the piezoelectric material. This implementation is further beneficial in that the magnitude of the output signal of the sensor is amplified by virtue of the lever arm that is formed by the distance from the point of impact of the material on the pin to the surface of the piezoelectric material to which it is attached. However, a major drawback of this implementation is that material often gets lodged in the sensor tube because of the pin obstructing a portion of the flow path. Another serious drawback of this design is that the pins often fail after a period of use due to being repeatedly struck by the particles in the air stream. These sensors also fail to accurately measure the amount of material flowing in the line because only very small amount of the material actually strikes the pin.
Yet another drawback to currently deployed piezoelectric sensor based systems, or any prior art system that places electronic sensors in or near to the stream of material, is that they require that the electronics associated with the sensors be replicated in every location where the sensors are installed. With current state-of-the-art air seeders employing a hundred or more tubes, the cost of these sensors, if deployed in every tube, can become a significant impediment to the deployment of the system.
In an attempt to save system costs, some air seeding systems will place sensors in only a small percentage of seed tubes and use those sensors to estimate the overall performance of the machine. An example of such a system is a material flow monitoring system that uses piezoelectric sensors mounted on top of secondary distribution manifolds in an air seeder to estimate the amount of material flowing into the manifold. In this embodiment, a piezoelectric sensor is caused to vibrate when material striking the inside top of the distribution manifold creates vibrations which are transmitted along a mounting bolt into the sensor. The principle of operation is that the amount of vibration transmitted up through the mounting bolt is directly proportional to the amount of material striking the manifold top. The assumption is that, if a material distribution line leaving the distribution manifold becomes blocked, the amount of material entering the manifold will decrease, causing the noise generated to decrease, which will indicate that a line is blocked. However, this system suffers from a number of serious shortcomings, principal of which is the lack of sensitivity of the system to partial blockages, whereby the flow to an individual tube may become partially restricted without substantially changing the total material flow into the distribution manifold; inasmuch as airflow and hence material flow will increase in the remaining distribution lines should one or more distribution lines become partially blocked. Another shortcoming of this system is that the amount of signal presented to the sensor can be highly variable from one manifold to the next on a single machine with multiple manifolds. Inasmuch as this system uses a common “sense line” and hence a common blockage sensing threshold, the level at which the system must be set in order to operate without causing blockage alarms must necessarily be less that the least sensitive node in the system. Therefore, the system can operate without alarm should a secondary distribution line become blocked or partially blocked in a manifold with a higher threshold requirement. Additionally, since this system is designed as a blockage monitoring system, it lacks both the sensitivity and granularity to be used as a material flow measurement system. Finally, this system, and others like it that do not use a sensor in each final run, are incapable of being used to help balance the flow of material across the final distribution runs of an air seeder system.
An important metric for measuring the balance of an air seeder system is the “Coefficient of Variation”. “Coefficient of Variation”, or CV, is a technical term used to describe the variability in the metering and distribution of material from the seed hopper throughout the seed tubes and into the soil. The CV is expressed as a percentage difference between the various final seed runs (known as “secondary seed tubes”) across the width of a seeder. The Prairie Agricultural Machinery Institute has published guidelines for CV values as its basis for rating the uniformity of distribution for seeding implements. These guidelines describe the rating scale as: a CV greater than 15% is unacceptable, a CV between 10% and 15% is acceptable, and a CV less than 10% is good.
If there is not a sensor capable of measuring material flow in every secondary seed tube, the only method of determining the CV for an air seeder is to run material through the system, collect the output of each of the final distribution tubes in separate containers, weigh the containers and compare their weights. This is obviously a laborious process which may have to be repeated multiple times as an individual is attempting to balance a system.
Even if sensors described in prior art are used in every secondary seed tube on an air seeder, these sensors are not, for the most part, designed to measure material flow, but rather are designed to detect lack of material flow (blockage). Assuming these sensors could be used to accurately measure material flow, there still is currently no efficient way of balancing the outputs of these tubes based on the data from the sensors. Some air seeding systems utilize hinged diverters or baffles at a branching point in a tube to direct more or less air flow down one of the two branches, but this can only affect the flow of the two branches of that particular tube in relation to each other, and does not correct any imbalance which may exist further downstream in the system.
Another problem common to all modern air seeders is the severe environmental conditions under which the equipment must operate. These conditions include extremes in vibration, dust, temperature, humidity, shock and moisture. As a consequence, electrical and electronics components utilized in these systems must be very robust or risk premature failure. It is undesirable to introduce components into this environment that by virtue of their electrical complexity are prone to failure. Unfortunately, that is precisely the situation with many of the flow monitoring systems in use today. Monitor systems of the present art, for the most part, employ sensors which are uniquely powered and addressed, resulting in hundreds of connections which are prone to failure due to the sever environmental conditions under which they must operate.
What is needed in the art is an inexpensive but accurate method of sensing and measuring the material flow in every secondary seed tube on an air seeder simultaneously, as well as a means for controlling or changing the air flow in each individual seed tube on the implement based on data derived from these sensors so as to properly balance the system and attain optimum seed uniformity.